Certain non-volatile memory devices utilize phase change technology to read and write data. The storage mechanism is typically a reversible change of state of a material or structure. For example, CD-Rewritable (CD-RW) and DVD-RAM optical disk drives use laser-induced structural phase change in an alloy layer on the disk to read and write data. The disk drives use laser energy to heat the material between amorphous and crystalline states to write data, and use the difference in reflectivity between the two states to optically read data.
Another class of phase change non-volatile memory devices utilize the electrical properties of the phase change material to read and write data, taking advantage of the difference in resistivity in the material in the different states. In the amorphous state, a small amount of current will pass, and in the crystalline state, the resistance of the material in that state will limit the current. Such devices may use an electric current to heat the material between amorphous and crystalline states.